1. Field of the Invention
The present invention relates generally to optical code division multiplexing (OCDM) systems and methodologies using passive integrated phase coders for multi-level security (MLS) in existing wavelength division multiplexing (WDM) networks.
2. Description of the Related Art
Faced with the demand for high capacity communication, there is growing interest in deploying WDM fiber optic networks. For many applications, for example in avionics, an optical physical layer that can support multi-level security is needed. Using different fibers (space division multiplexing) is an obvious choice, but is costly because it requires additional fiber infrastructure. In wavelength division multiplexing networks, different security levels are carried on the same fiber, but on different optical channels. However, concerns remain regarding wavelength division multiplexing enabled multi-level security being susceptible to eavesdropping through inter-window cross talk as well as through more conventional means of eavesdropping.
Conventional optical eavesdropping can include, for example, physical tapping of the fiber.
An example of eavesdropping via crosstalk is described below.
In a dense wavelength division multiplex (DWDM) system, each of several input signals enter a DWDM node or network element and is assigned or converted to a specific wavelength, typically, in the 1550 nanometer (nm) band. After wavelength conversion, each individual signal wavelength or channel is then multiplexed by wavelength division multiplexing and transmitted onto the same fiber. Consequently, a single fiber carries more than one wavelength. In fact each wavelength carried by a DWDM system may be considered a virtual fiber.
The signal carried on the virtual fibers of DWDM systems may be susceptible to eavesdropping of a form that is not possible if the signals are on separate fibers. In DWDM systems different channels travel through the same fiber and the same components. As a result of cross-talk, nonlinearity, etc., at the receiving end, there may be a residual of signal(s) from other channels that can be isolated, amplified and detected.
The potential for eavesdropping may be better appreciated by reference to FIG. 1 where there is depicted a receiving node 100 in a DWDM network. Receiving node 100 may be an optical demultiplexer or add drop multiplexer, a wavelength converter, or an optical cross-connect that serves as a drop off or interchange point for one or more channels. FIG. 2 shows, on a logarithmic scale, the optical spectrum of channel 10 in FIG. 1 as it dropped from node 100. As FIG. 2 shows, although the goal was to drop only channel 10, channel 11 is clearly visible. In FIG. 3, a filter is used to reduce the optical signal to noise ratio (OSNR) for channel 10. As FIG. 3 shows, channel 11 is still present with enough power to be recoverable. In fact, in FIG. 4, the channel 10 transmitter has been turned off and as FIG. 4 shows there is a significant amount of residual power still present from channel 11. Results similar to those shown in FIG. 4 have also been achieved by introducing a second filter to attenuate channel 10 in the received spectrum. In either case, in FIG. 4, channel 11 is leaked with large enough OSNR to be recoverable after optical amplification. Accordingly, the user of channel 10 may be able to recover channel 11 without the network operator ever knowing of the breach in security. On another level, residual power from each channel may be available on all the channels thereby providing for security akin to having a party line.